Due to recent improvement of the performance of physical devices (hereinafter, also referred to as physical machines), study has been undertaken on a virtualization technique of aggregating a plurality of virtual devices (hereinafter, also referred to as virtual machines) into a single physical machine study. The virtualization technique enables a service to be provided from an application program (hereinafter, also referred to as an application) installed in each virtual machine, with the physical machines being allocated to a plurality of virtual machines by virtualization software (hypervisor) for example.
In a virtual machine which is under operation, migration of a content (hereinafter, referred to as a disk image) constituting a virtual machine to a virtual machine that is generated in another physical machine may be performed. A service provider using the virtual machine (hereinafter, also simply referred to as a service provider) performs the migration to migrate the disk image in the virtual machine generated in the physical machine with the CPU use rate close to 100% to another physical machine with lower CPU use rate, for example. Thus, the provider can balance processing loads among the physical machines (see, for example, Japanese Patent Application Publication No. 2010-39685 and Japanese Patent Application Publication No. 2011-134320).